I just read an article in the Huffington Post about why we don't need negative reviews, and I have to say I disagree. It's not only that I agree with Maureen Dowd that to only post positives is to act as a marketing shill for the publishers, but I also think that it proves the reviewer's objectivity which yes, is often in question. Personally, I have found negative reviews to be of great help in the past. When I used to select books for major retailers, they had strict guidelines to adhere to and often books that did not meet those, (suggested punishments instead of only praise for training dogs, claimed to be an organic cookbook but suggested brands of food known for using GMOs) I found out about the issues in the negative reviews. I also think that I am entitled to my opinion. And I know it likely won't deter many people from reading The Ask, but I truly hated it and if someone has very similar tastes to mine, I hope to prevent them from the torture I found that book to be. I am not writing a negative review to hurt Sam Lipsyte's feelings; in fact I doubt he cares at all about my opinion, but I did not like the book at all and I can say so.
Now it's true that I am simply writing for my own blog and that is different than reviewing for a media outlet, however I think even then, readers are entitled to be forewarned when a mystery introduces the killer in the second-to-last chapter, when a romance novel doesn't have a happy ending, or when a history book will put you to sleep with poor research. Reviews are done in the service of readers, not authors or publishers, and so an occasional warning is a good thing.
Also, what some people might think of as a good review, can be bad to others. I personally know I have several red flags that will make me drop a book like a hot potato and run away screaming, and they will surprise you. They include the words lyrical, poetic, atmospheric, and earnest. They include comparisons to Gabriel Garcia Marquez, David Mitchell, or Arundhat Roy. A reviewer might love a book and use a lot of the above adjectives, compare it to Charles Frazier or William Faulkner, and I'll be gagging into the nearest trashcan. So even when we think we've written a positive review, it might be taken as negative by some people. Since there's no way to prevent that, why should we dissemble and pretend to like everything? Just be straightforward, leave the author out of it, and talk about why you didn't like it (no fair saying you hate a book without backing it up) and who knows--one person's negative review might be another's positive. I do have a friend whose book tastes are so opposite to mine that I have read books specifically because she told me she hated them.
As long as a review is honest and fair, I think there's plenty of room for negative reviews in the world.